Thyrozol Special Precautions

Thyrozol should not be used in patients with history of mild hypersensitivity reactions (eg, allergic rashes, pruritus). Thiamazole should only be used in short-term treatment and with careful monitoring in patients with large goiters with constriction of the trachea because of the risk of goiter growth.
Agranulocytosis has been reported to occur in about 0.3-0.6% of cases. Therefore, patient must be informed prior to the start of therapy of the related symptoms (stomatitis, pharyngitis, fever). It usually occurs during the 1st weeks of treatment, but may still become apparent some months after the start of therapy and upon its reintroduction. A close monitoring of blood count is recommended before and after initiation of therapy especially in cases with preexisting mild granulocytopenia.
In the case that any of these symptoms occured, especially during the 1st weeks of treatment, patients should be advised to contact their physician immediately for a blood count. If agranulocytosis is confirmed, Thyrozol must be discontinued.
Other myelotoxic adverse reactions rarely occur in the recommended dose range. They have frequently been reported in connection with very high doses of thiamazole (about 120 mg daily). These dosages should be reserved for special indications (severe courses of disease, thyrotoxic crisis). Occurrence of damage to the bone marrow during treatment with thiamazole requires discontinuation of the medication and if necessary, switching to an antithyroid drug of another substance group.
Excess dosage can lead to subclinical or clinical hypothyroidism and goiter growth due to thyroid-stimulating hormone (TSH) increase. Therefore, the dose of thiamazole should be reduced as soon as a euthyroid metabolic condition is achieved and if necessary, levothyroxine should be given additionally. It is not useful to discontinue thiamazole altogether and to continue with levothyroxine only.
Goiter growth under therapy with thiamazole in spite of suppressed TSH is a result of the underlying disease and cannot be prevented by additional treatment with levothyroxine.
Achievement of normal TSH levels is crucial to minimize the risk of occurrence or deterioration of endocrine orbitopathy. However, this condition is frequently independent of the course taken by the thyroid disease. Such a complication itself does not constitute a reason to change the adequate treatment regimen and is not to be regarded as an adverse reaction of appropriately performed therapy.
At a low percentage, late hypothyroidism can occur after antithyroid therapy without any additional ablative measures. This is probably not an adverse drug reaction, but to be regarded as inflammatory and destructive processes in the thyroid parenchyma due to the underlying disease.
The reduction in the pathologically increased energy consumption in hyperthyroidism can lead to a (generally desired) gain in body weight during treatment with thiamazole. Patients are to be informed that their energy consumption normalizes along with the improving clinical picture.
Thyrozol contains lactose; therefore, patients with rare hereditary galactose intolerance, lactase deficiency or glucose-galactose malabsorption should not take Thyrozol.
Effects on the Ability to Drive or Operate Machinery: Thiamazole does not affect the capability to drive a vehicle or to operate machinery.
Use in pregnancy & lactation: In general, pregnancy has a positive effect on hyperthyroidism. Nevertheless, treatment of hyperthyroidism is often required especially in the 1st months of pregnancy. Untreated hyperthyroidism during pregnancy may lead to serious complications eg, premature birth and malformation. However, hypothyroidism caused by treatment with inappropriate thiamazole doses is also associated with a tendency to abortion.
Thiamazole passes the placental barrier, and in fetal blood, reaches concentrations equal to those found in maternal serum. At an inappropriate dosage, this may lead to goiter formation and hypothyroidism in the fetus as well as to reduced birth weight. There have been repeated reports of partial aplasia cutis on the head of neonates born to women treated with thiamazole. This defect healed spontaneously within a few weeks.
In addition, a certain pattern of diverse malformations has been associated with high-dose thiamazole therapy during the 1st weeks of pregnancy eg, choanalatresia, oesophageal atresia, hypoplastic nipples, delayed mental as well as motor development. In contrast, several case studies on prenatal thiamazole exposure have neither revealed any morphological development disorders nor impact on thyroid development or the physical and intellectual development of the children.
Since embryotoxic effects cannot be completely excluded, Thyrozol may only be administered during pregnancy after carefully weighing the benefit-risk ratio and only at the lowest still effective dose level without additional administration of thyroid hormones.
Thiamazole passes into breast milk where it can reach concentrations corresponding to maternal serum levels, so that there is a risk of hypothyroidism developing in the infant.
Breastfeeding is possible during thiamazole treatment; however, only low doses up to 10 mg daily may be used without additional administration of thyroid hormones. The infant's thyroid function is to be monitored regularly.